Particle accelerator tubes with circumferentially corrugated accelerator electrodes



1958 w. D. ALLEN ETAL 3,363,125

PARTICLE ACCELERATOR TUBES WITH CIRCUMFERENTIALLY CQRRUGATED ACCELERATOR ELECTRODES Filed Aug. 23, 1965 3 Sheets-Sheet 1 1968 w. D. ALLEN ETAL 3,363,125

PARTICLE ACCELERATOR TUBES WITH CIRCUMFERENTIALLY CORRUGATED ACCELERATOR ELECTRODES Filed Aug. 25, 1965 3 Sheets-Sheet 2 Jan. 9, 1968 w. D. ALLEN ETALY 3,363,125

PARTICLE ACCELERATOR TUBES WITH CIRCUMFERENTIALLY CORRUGATED ACCELERATQR ELECTRODES Filed Aug. 23, 1965 3 Sheets-Sheet 5 FIG. 5.

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Q L: u L QHM l' [I [I I: I 1 l I 1 '/E20 1, 3 521 H u [1 I w H I I H I Patented Jan. 9, 1968 3,363,125 PARTICLE ACCELERATOR TUBES WITH CIRCUM- FERENTIALLY CORRUGATED ACCELERATOR ELECTRODES William Douglas Allen, Burcot, Ralph Herbert Vernon Mordaunt Dawtou, Boars Hill, and Donald Anthony Gifford Broad, Abingdon, England, assiguors to Science Research Council, London, England Filed Aug. 23, 1965, Ser. No. 481,593 Claims priority, application Great Britain, Aug. 28, 1964, 35,425/64 4 Claims. (Cl. 313-63) This invention relates to particle accelerator tubes suitable for use with electrostatic generators, for example of the Van de Graatf type.

The performance of a particle accelerator comprising an electrostatic generator and an accelerator tube is normally limited by the quality of the accelerator tube, at high vacuum, through which the particles (positive or negative ions) are accelerated. The limitation manifests itself through the X-rays generated by the electrons which stream back to the positive high voltage electrode through the evacuated region. The X-rays, which can be readily detected outside the pressure vessel enclosing the electrostatic generator, cause large ionisation currents in the high pressure gas in the vessel, this gas constituting the insulator against high voltage breakdown. As both the electron currents and the resultant X-rays rise steeply above a given voltage threshold, machine performance is commonly limited by the ionisation currents liberated by the X-rays in the gas of the pressure vessel.

The origin of this so-called electron loading is obscure and although it is clear that some electron multiplication eifect is occurring the phenomenon has not been elucidated.

The specification of patent application Ser. No. 214,748, now Patent No. 3,304,504, issued Feb. 14, 1967, discloses an accelerator tube which is arranged such that the electron loading is very considerably reduced. In general terms this tube comprises a plurality of electrodes of which the planar region adjacent the beam is normal to the tube axis, followed by a plurality of electrodes of which the planar region adjacent the beam is tilted relative to a plane normal to the tube axis, the azimuthal direction of tilt of successive electrodes being incrementally varied to form at least one pair of consecutive 360 spirals spiralling in opposite directions, and the increment being such that the net displacement and angular deviation of the beam in traversing the pair of spirals are substantially zero.

As stated above, the electron loading of this accelerator tube is considerably reduced, as compared that is with an accelerator tube in which all the electrodes have the planar region adjacent the beam normal to the tube axis. In one particular example the X-rays produced were less by a factor of ten.

There is however a residual difiiculty which shows itself as beam wobble. This is a small rapid fluctuation in the beam due to local inductive effects of the chargecarrying belt of the electrostatic generator.

An object of the present invention is to provide a particle accelerator tube in which this difiiculty may be reduced.

According to the present invention a particle accelerator tube comprises a plurality of electrodes whereof the region adjacent the beam has its general plane normal to the tube axis but has its inner edge circumferentially corrugated, each electrode being rotated about the tube axis by the same angle relative to the previous electrode so that the net tangential deflection of the beam is substantially zero.

Preferably, the accelerator tube comprises a first plurality of electrodes whereof the planar region adjacent the beam is normal to the tube axis, followed by a second plurality of electrodes whereof the region adjacent the beam has its general plane normal to the tube axis but has its inner edge circumferentially corrugated, each electrode of the second plurality of electrodes being rotated about the tube axis by the same angle relative to the previous electrode so that the net tangential deflection of the beam due to the second plurality of electrodes is substantially zero, followed by a third plurality of electrodes whereof the planar region adjacent the beam is tilted relative to a plane normal to the tube axis, the azimuthal direction of tilt of successive electrodes being incrementally varied to form at least one pair of consecutive 360 spirals spiralling in opposite directions, and the increment being such that the net displacement and angular deviation of the beam in traversing the third plurality of electrodes are subsequently zero.

A particle accelerator tube in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a sectional elevation of a tilted electrode,

FIGURE 2 is a plan view of the electrode of FIG- URE 1,

FIGURE 3 is a sectional elevation of a corrugated electrode,

FIGURE 4 is an isometric view of the electrode of FIGURE 3, and

FIGURE 5 is a simplified sectional elevation of the accelerator tube.

Accelerator tubes have commonly been made with electrodes in the general shape of dishes in which the planar area adjacent to the 'beam is normal to the beam. In the present accelerator tube some of the electrodes are of this form but some, whilst generally of this form, have the planar area modified. Some of the electrodes, referred to herein as corrugated electrodes, have their inner edges deformed, and some, referred to herein as tilted electrodes, have this planar area at an angle to the plane normal to the tube axis. The modified electrodes will be referred to as normal electrodes.

A tilted electrode is shown in FIGURES 1 and 2 to which reference is now made. The electrode includes the planar area 1 adjacent a hole 2, through which the beam passes. The desired axis 3 of the beam is indicated by an arrow, the planar area 1 making an angle 6 with the normal to the axis 3. Two additional holes 4 are provided to make the accelerator tube easier to pump out. The arrow 5 in FIGURE 2 indicates the direction of tilt.

A corrugated electrode is shown in FIGURES 3 and 4 to which reference is now made. The electrode includes the area 6 which is initially planar but which becomes deformed to some extent when the inner edge of the electrode is circumferentially corrugated. The corrugation is such that the inner edge undulates in a direction parallel to the axis 3 of the beam. Thus, three symmetrically disposed points 7 on the inner edge are displaced out of the general plane of the area 6 in an upward direction and three points 8 disposed symmetrically between each pair of points 7 respectively are displaced by the same distance out of the general plane of the area 6 in a downward direction. (In fact the number of points 7 and 8 need not be three, although this is a very convenient number.)

Reference will now be made to FIGURE 5 which indicates the electrode arrangement in the accelerator tube 9. The first group of electrodes E1 to E20 are normal electrodes and in the length of the accelerator tube 9 occupied by these electrodes E1 to E20 the particles (positive ions) in the beam are accelerated to about 0.8 electron megavolt.

The next group of electrodes E21 to E60 are corrugated electrodes and in the length of the accelerator tube 9 occupied by these electrodes E21 to E60 the particles are accelerated to about 2.4 electron megavolts.

Because of the corrugation of the inner edges of the electrodes E21 to E60, any electron which originates at or near the inner edge of an electrode will be in a tangential electric field and will therefore tend to be accelerated still further away from the axis 3 of the beam and strike an electrode a comparatively short distance along the accelerator tube 9. As such electrons travel only a short distance before striking an electrode their maximum energies are far less than they would attain by acceleration throughout the length of the accelerator tube 9. The X-ray yield and penetration, which increase rapidly with increasing electron energy, is therefore considerably reduced.

This tangential field decreases rapidly with decreasing radius, and the beam itself is virtually unaffected by the corrugations of the inner edges of the electrodes E21 to E60. If, therefore, there are variations in the potentials of the electrodes E21 to E60 due to inductive pick-up from the charge-carrying belt of the electrostatic generator, the position of the beam is substantially unaffected.

There is a tendency for a small net tangential deflection to be impressed on ions on the outskirts of the beam, but this tendency is suppressed by rotating the electrodes E21 to E60 down the accelerator tube 9. Each of the electrodes E21 to B60 is rotated about the axis 3 of the accelerator tube 9 by the same angle relative to the previous electrode so that in passing from the electrode E21 to the electrode E60 there is a 360 spiral. (Strictly only a 120 spiral is necessary, as this is sufficient to displace each point 7 to the angular position of one of the adjacent points 7 in passing from electrode E21 to electrode E60.)

The second group of electrodes E61 to E144 are tilted electrodes and in the length of the accelerator tube 9 occupied by these electrodes E61 to E144 the ions are accelerated from about 2.4 to 6 electron megavolts.

If all the tilted electrodes E61 to E144 were parallel, the resultant deviation of the beam from the desired axis 3 would not be negligible. To avoid this deviation the electrodes E61 to E72 are spiralled in much the same way as the electrodes E21 to E60 so that the direction of deflection is also a spiral. If the azimuthal direction of tilt of electrode E61 is taken as oc=O (the datum), then with a suitable rate of change of a between successive electrodes the net deflection of the beam can be made negligible at electrode E72 where a=360. The arrow 10 in FIGURE 2 indicates the position of the arrow for an electrode whose azimuthal direction of tilt is a relative to the datum.

There must be taken into account, however, not only the deflection, that is the angular deviation of the beam from the desired axis 3, but also its displacement from the axis 3. It can be shown that for the accelerator tube described this displacement is quite large at electrode E72 (about 0.06 inch, in the azimuthal direction of a=90). The displacement can be corrected by using a second spiral spiralling in the opposite direction. Thus in FIGURE 5 the first spiral ends at electrode E72 where u=+360; the second spiral begins at electrode E73 and ends at electrode 84 where a=360. By suitable varying the rate of change of on between successive electrodes as described in the above-mentioned specification, the net displacement as well as the net deviation can be made negligibly small at electrode E84.

There then follows two further pairs of 360 spirals spiralling in opposite directions to one another and extending from E85 to electrode E and from electrode E111 to electrode 14-4. The rate of change of a between successive electrodes in each of these spirals is also varied in the way described in the abovementioned specification so that the net displacement and the net deviation is negligibly small at electrode E144, that is at the end of the accelerator tube 9.

We claim:

1. A particle accelerator tube comprising a plurality of annular electrodes arranged along the axis of the tube wherein the innermost region of each electrode has its general plane normal to said axis but has its inner edge circumferentially corrugated, each electrode being angularly displaced about said axis by the same angle relative to the previous electrode so that the net tangential deflection of the beam is substantially zero.

2. A particle accelerator tube in accordance with claim 1 wherein the inner edge of each of said plurality of electrodes is circumferentially corrugated so that at least two symmetrically disposed points on said edge are displaced out of said plane in one direction parallel to said axis and a similar number of points disposed symmetrically between each pair of first-mentioned points respectively is displaced by the same distance out of said plane in the opposite direction, the intervening portions of said edge smoothly connecting said points.

3. A particle accelerator tube in accordance with claim 2 wherein the number of said points displaced in each direction is three.

4. A particle accelerator tube comprising a first plurality of annular electrodes arranged along the axis of the tube wherein a planar innermost region of each electrode is normal to the said axis, followed by a second plurality of electrodes wherein an innermost region of each electrode has its general plane normal to said axis but has its inner edge circumferentially corrugated, each electrode of the second plurality of electrodes being angularly displaced about said axis by the same angle relative to the previous electrode so that the net tangential deflection of the beam due to the second plurality of electrodes is substantially zero, followed by a third plurality of electrodes wherein a planar innermost region of each, electrode is tilted relative to a plane normal to said axis, the azimuthal direction of tilt of successive electrodes being in rementally varied to form at least one pair of consecutive 360 spirals spiralling in opposite directions, and the increment being such that the net displacement and angular deviation of the beam in traversing the third plurality of electrodes are substantially zero.

References Cited UNITED STATES PATENTS 3,308,323 3/1967 Van de Graatf 313-63 DAVID I. GALVIN, Primary Examiner, 

1. A PARTICLE ACCELERATOR TUBE COMPRISING A PLURALITY OF ANNULAR ELECTRONS ARRANGED ALONG THE AXIS OF THE TUBE WHEREIN THE INNERMOST REGION OF EACH ELECTRODE HAS ITS GENERAL PLANE NORMAL TO SAID AXIS BUT HAS ITS INNER EDGE CIRCUMFERENTIALLY CORRUGATED, EACH ELECTRODE BEING ANGULARLY DISPLACED ABOUT SAID AXIS BY THE SAME ANGLE RELATIVE TO THE PREVIOUS ELECTRODE SO THAT THE NET TANGENTIAL DEFLECTION OF THE BEAM IS SUBSTANTIALLY ZERO. 